The present invention relates to a method and apparatus for determining the draw down characteristics of a well. More particularly, the invention relates to a technique wherein both static and dynamic water table levels are measured by means of an absolute pressure transducer immersed in the bore hole, independent of any casing or tubing structure, and a direct reading measuring circuit located at the surface is used to derive the draw down of the well. The draw down data is then used in conjunction with water pressure and water flow data to provide a monitoring system for water wells.
One of the tests that is paramount to determining the productivity of a water well is measuring the influx of water to the well during pumping. The influx is directly indicative of the ability of the surrounding soil formation to give up the water it contains. Poor water producing formations may contain a plentiful reservoir of water but yield it at rates which render wells located therein of low productivity. While the water well industry has long been aware of the importance of draw down as a key factor indicating the quality of a well, the methods for measuring it have remained largely manual. Two other factors which are well known as being required to evaluate the influx of a well are the discharge pressure and volume of flow of the water being produced. As a result of the relative difficulty of having a measuremment of well draw down quickly and easily available, there is a tendency to overlook the importance of having frequent measurements of the parameter. A number of foreign countries have rigid requirements calling for measurements of a water well's productivity upon completion of drilling. In addition to the importance of having the influx characteristics of a well known for evaluating the production quality, the three factors mentioned above are also invaluable for making rapid and accurate diagnosis of fault symptoms, and for planning of preventive maintenance of the well and its facilities.
The basic method for measuring the draw down of a well involves an operator lowering an electrical contact on a plumb bob and measuring the length thereof when contact with the water is made. As the water level changes, the operator must try to follow the change with the plumb bob. Obviously, this is very difficult to do and yields only approximations if the level is changing rapidly.
Other representative methods and devices for measuring draw down have been used in the past, however, most prior art techniques lack automaticity, or accuracy or involve multistep processes and calculations. Typical prior art approaches are found in U.S. Pat. Nos. 3,321,965 to C. R. Johnson et al; 3,780,574 to Miller; and 3,737,728 to Fitzpatrick. Many of the prior art approaches to measurements of draw down are derived from oil well drilling and measuring techniques, which are not entirely applicable to the water well industry. For the most part this is due to the fact that oil well operations generally are carried out in structural environments wherein pressures encountered in the well bore are due to geological conditions. In direct contrast, the water well environment is one in which gravity is the predominant factor, hence the measurement of draw down is done under conditions wherein the depth of a column of water is directly and linearly related to the hydrostatic pressures encountered at that depth.